Always worth checking here when planning stuff that involves dealing with any government department.
Also worth remembering that if you go for your extension 1-2 weeks before your expiry date, you don't lose any time; the extension is added to your *existing* date, not the date you applied for the extension.
There's no *official* limit to air entries. On paper, you can do it forever.
However, the more visa-exempt entry stamps you have in your passport, back-to-back, the greater the likelihood that you'll be pulled aside at immigration and interrogated as to what you're doing here, and potentially denied entry, or worst-cast-scenario deported back to your country of origin.
When that point comes will vary from officer to officer.
Some people get questioned after their second or third entry stamp.
Others get away with it for well over a year.
"How long can I stay here on short-term stays" is one of those "How long is a piece of string" questions.
You can do it until you can't; until they tell you "No More".